1. Field of the Invention
The present invention relates to an extreme ultra violet (EUV) light source apparatus to be used as a light source of exposure equipment.
2. Description of a Related Art
Recent years, as semiconductor processes become finer, photolithography has been making rapid progress to finer fabrication. In the next generation, microfabrication of 100 nm to 70 nm, further, microfabrication of 50 nm or less will be required. Accordingly, in order to fulfill the requirement for microfabrication of 50 nm or less, for example, the development of exposure equipment is expected by combining an EUV light source generating EUV light having a wavelength of about 13 nm and a reduced projection reflective optics.
As the EUV light source, there are three kinds of light sources, which include an LPP (laser produced plasma) light source using plasma generated by applying a laser beam to a target (hereinafter, also referred to as “LPP type EUV light source apparatus”), a DPP (discharge produced plasma) light source using plasma generated by discharge, and an SR (synchrotron radiation) light source using orbital radiation. Among them, the LPP type EUV light source apparatus has advantages that extremely high intensity close to black body radiation can be obtained because plasma density can be considerably made larger, that light emission of only the necessary waveband can be performed by selecting the target material, and that an extremely large collection solid angle of 2π steradian can be ensured because it is a point source having substantially isotropic angle distribution and there is no structure surrounding the light source such as electrodes. Therefore, the LPP light source is considered to be predominant as a light source for EUV lithography requiring power of more than several tens of watts.
Here, a principle of generating EUV light in the LPP type EUV light source apparatus will be explained. By applying a laser beam to a target material supplied into a vacuum chamber, the target material is excited and plasmarized. Various wavelength components including EUV light are radiated from the plasma. Then, the EUV light is reflected and collected by using an EUV collector mirror that selectively reflects a desired wavelength component (e.g., a component having a wavelength of 13.5 nm), and outputted to an exposure unit. For the purpose, a multilayer film in which thin films of molybdenum (Mo) and thin films of silicon (Si) are alternately stacked (Mo/Si multilayer film), for example, is formed on the reflecting surface of the EUV collector mirror.
FIG. 7 shows a droplet target generating device and a part around the device in a conventional EUV light source apparatus. As a target material, for example, tin (Sn) melted into the liquid state, lithium (Li) melted into the liquid state, or a material formed by dissolving colloidal tin oxide fine particles in water or a volatile solvent such as methanol is used.
The target material introduced into a target tank 101 is pressurized with a pure argon gas or the like, for example, and a jet of the target material is ejected from an injection nozzle 102 attached to the leading end of the target tank 101 and having an inner diameter of several tens of micrometers. When regular disturbance is provided to the jet by using a vibrator (not shown) attached to the injection nozzle 102 or near the injection nozzle 102, a jet part 1a of the target material immediately breaks up into droplets 1b having homogeneous diameters, shapes, and intervals. The method of generating the homogeneous droplets in this manner is called a continuous jet method.
The generated homogeneous droplets 1b move within a vacuum chamber 100 according to the inertia when the jet is ejected from the injection nozzle 102, and a laser beam radiated from a CO2 laser or YAG laser, for example, is applied thereto at a laser application point. Thereby, the target material is plasmarized and EUV light is radiated from the plasma. The droplets that have not irradiated with laser are collected by a target collecting unit 106 provided at the opposite side to the injection nozzle 102 with the laser application point in between.
However, in the conventional technology, the stability of the positions of droplets are gradually lost and the positions become unstable before the droplets reach the laser application point, and variations in positions are increased especially in the traveling direction of the droplets. As a result, the laser beam is no longer applied to the droplets constantly in the same condition, and there is a problem that the intensity of the radiated EUV light varies and, in the worst case, the laser beam is not applied to the droplets and no EUV light is generated. The trouble due to instability in positions of droplets becomes significant as the inner diameter of the injection nozzle 102 becomes smaller and the diameters of the droplets and intervals between the droplets become smaller.
FIG. 8 is a photograph of droplets generated by the droplet target generating device shown in FIG. 7. As the target material, melted tin is used. As shown in FIG. 8, the turbulence occurs in the positional stability of droplets at the laser application point, and the intervals between droplets are inhomogeneous and plural droplets are combined in some locations.
As one method of solving the problem, it is conceivable to apply laser beam to the droplets in a point where the positional stability of droplets is in a relatively good condition, that is, a point at a flying distance from the injection nozzle 102 is short (e.g., a point at a distance of about 50 mm from the injection nozzle 102). However, since the laser poser to be used in the EUV light source is 10 kW or more, the heat input to the injection nozzle 102 or the part around the nozzle is greater, the stably droplet generation is not maintained, and consequently, the performance of the EUV light source is deteriorated.
As a related technology, U.S. Patent Application Publication US 2006/0192154 A1 discloses EUV plasma formation target delivery system and method. The target delivery system includes: a target droplet formation mechanism comprising a magneto-restrictive or electro-restrictive material, a liquid plasma source material passageway terminating in an output orifice; a charging mechanism for applying electric charge to a droplet forming jet stream or to individual droplets exiting the passageway along a selected path; a droplet deflector positioned between the output orifice and a plasma initiation site, for periodically deflecting droplets from the selected path, a liquid target material delivery mechanism comprising a liquid target material delivery passage having an input opening and an output orifice; an electromotive disturbing force generating mechanism for generating a disturbing force within the liquid target material, a liquid target delivery droplet formation mechanism having an output orifice; and/or a wetting barrier around the periphery of the output orifice. However, US 2006/0192154 A1 does not particularly disclose improvements in positional stability.
Further, HEINZL et al., “Ink-Jet Printing”, ADVANCES IN ELECTRONICS AND ELECTRON PHYSICS, U.S., Academic Press, 1985, Vol. 65, pp. 91-171 describes an explanation about the continuous jet method. According to HEINZL et al., the transformation from laminar to turbulent-like jet flow depends on the aspect ratio L/d of the nozzle, where “L” is the length and “d” is the diameter. Further, laminar-flow jets break up into a train of drops at some point due to surface tension. This is due to the fact that the surface energy of a liquid sphere is smaller than that of a cylinder having the same volume. Therefore, a jet of fluid column having, for example, a cylindrical shape is inherently unstable and will eventually transform itself into drops having a spherical shape (page 132).